Potato and Forest Mushroom Lasagna with Nage Butter Sauce

I think this dish can challenge any lasagna made from conventional pasta, and the technique of making tender potato sheets is really quite easily mastered once you have done it two or three times. The recipe calls for four different types of mushrooms, but in a pinch you could use 3/4 pound of one kind of mushroom, although, of course the flavor will not be as rich. You could cook the potato layer earlier in the day and leave them, covered with the parchment paper, until just before assembl